Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescence element (hereinafter, referred to as an inorganic EL element) and an organic electroluminescence element (hereinafter, referred to as an organic EL element).
An inorganic EL element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element
On the other hand, an organic EL element is an element provided with a constitution comprising an light emitting layer containing a light emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the emitting layer to be recombined, resulting light emission utilizing light release (fluorescence-phosphorescence) at the time of deactivation of said exciton; the light emission is possible at a voltage of approximately a few to a few tens volts, and an organic EL element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Moreover, an organic EL device has a distinctive feature of being a surface light, which is different from the main light sources, for example, a light-emitting diode and a cold cathode tube having been conventionally used. As applications which can effectively utilize this property, there are a light source for illumination and a back light of various displays. Especially, it is suitable to use as a back light of a liquid crystal full color display the demand of which has been increased remarkably in recent years.
When an organic EL element is used as a light source for illumination or a back light of a display, the used light source is required to emit a white light or an electric bulb color (hereafter they are called as a white color).
The following ways are known to achieve in obtaining a white light: a method to laminate B/G/R three light emitting layers or B/Y two light emitting layers which are in complementary color relation (for example, refer to Patent document 1); a method in which multicolor emission pixels, for example, three color of blue, green, and red, are provided by coating separately, and these pixels are made to emit a light simultaneously to obtain a white light by mixing (for example, a combination of a blue luminescent material and a color conversion fluorescent dye); and a method of adjusting several luminescent materials each having a different emission wavelength in one element to obtain a white light by mixing.
However, there was a problem that a luminescent color will change when laminating a plurality light emitting layers each emitting a different color light, because a light-emitting position would shift by the change of the driving electric current or by the chronological change in continuous driving. Moreover, the method of providing multicolor emission pixels by coating separately has a problem of using complicated manufacturing processes, such as positioning a mask, and its production yield is not good. Further, there is a problem that luminous efficiency is low by a color conversion mode.
A method of stopping the slippage of a light-emitting position is proposed to resolve these problems by making all the luminescent materials intermingled in a single light emitting layer. However, if luminescent materials are intermingled, energy transfer will occur by the difference in the luminescence energy level of each luminescent material.
Moreover, there is described a method of improving efficiency using the energy transfer between the luminescent materials which exist together in the same layer (for example, refer to Patent document 2). However, according to this method, even if the luminescent materials in which luminescent colors differ are mixed, only one luminescent material emits light, and it is not suitable for obtaining a white light.
That is, in order to obtain desirable white luminescence by a single light emitting layer, white luminescence cannot be obtained by making the ratio of the luminescent materials to be the same as incorporated in a multilayered constitution. The content of the luminescent material having a low luminescence energy level must be adjusted to be a small quantity compared to the content of the luminescent material having a high luminescence energy level, and control of material ratio is difficult when the element is produced by a vacuum evaporation.
On the other hand, as a production method of an organic EL element, there are wet processes (for example, a spin coat method, a cast method, an ink-jet method, a spray method, and a printing method). In recent years, a production method of a wet process has been attracted attention since it does not require a vacuum process and continuous production is easily done. In a wet process, it can be formed a light emitting layer having a desired composition by adjusting the material mixing ratio at the time of coating solution preparation. It has an advantage when the light emitting layer containing materials having greatly different mixing ratio is formed.
Moreover, using a polymer by a wet process in organic EL material is already known widely (for example, refer to Patent documents 3 and 4), and it is recognized as a useful technique. Furthermore, the organic electroluminescence element (it is also called an organic EL element) using a polymeric material having a specific weight average molecular weight is introduced as a well-known technique (for example, refer to Patent document 5).
By using a polymer as an organic EL material for a wet process with reference to these patent documents, it was thought that a very useful element would be obtained and development was investigated. However, it became clear that there was a new problem which was not disclosed in patent documents. That is, even if a polymer material was used, it was difficult to constitute the laminated structure continuously by a wet process. When the following layer is applied on a polymer material, a part of polymer material will be dissolved into the solvent used for the next application, and irregularity will arise in a membrane interface. The difference in the coating thickness by the irregularity affects the properties. When an electric current is passed in an element, a non-uniformity arises in electron flow, a rectification ratio becomes worse, and there will appear the phenomenon in which an element will not shine suddenly. Moreover, when a higher molecular weight was advanced too much in order to decrease the solubility in the solvent used for a next layer, it was suggested that there were deteriorate effects to external extraction quantum efficiency and a lifetime.